Large Photoelasticity in Topological Antiferromagnet Mn${}_3$Sn Studied by Coherent Acoustic Phonon

We investigate the relation of electronic and optical properties to ultrafast strain in topological antiferromagnet Mn${}_3$Sn thin films using near-infrared femtosecond laser pulses. Coherent acoustic phonon oscillations are generated and clearly identified with a remarkably large amplitude exceeding 1% in differential transmission. Quantitative analysis reveals that near-infrared photoelastic coefficient in Mn${}_3$Sn is several times larger than that in typical materials. These results establish a quantitative approach for understanding the large optical responses to ultrafast strain in the topological kagome antiferromagnet, suggesting potential opto-spintronic and optoacoustic applications even at telecommunications wavelength.